Breast cancer is believed to be related to an affluent, high-fat diet, yet few case-control studies of this cancer have been able to demonstrate the strong dietary associations presumed to exist. Since native Japanese and Chinese women have breast cancer mortality rates approximately one-fifth those of white American women, and breast cancer rates rise in successive generations among Asian families who migrate to the United States, and Asian-American study population might provide a sufficiently heterogeneous risk of breast cancer among women of Chinese, Japanese, and Filipino heritage in the San Francisco-Oakland SMSA (Standard Metropolitan Statistical Area), the Los Angeles SMSA, and Oahu, Hawaii-the only three areas of the United States with large number of Asian-American residents and population-based cancer registries. It has been hypothesized that diet during childhood and adolescence may be more crucial than adult diet in determining the risk of breast cancer. To assess the role of early as well as adult diet, the subjects selected were 55 years or less so that both they and their mothers could be interviewed about the subjects' early diet.